1. Field of the Invention
This invention relates to the field of fast switching input-output couplers for high rate data transfer buses. The working frequencies of this type of coupler are situated in the range from 100 to 200 MHz. It is possible to utilise CMOS technology so as to favour low heat dissipation. In addition, this type of coupler is compatible with emitter coupled logic technology ("ECL"), as the working voltage is defined at 0.8 V.
2. Description of the Related Art
Several couplers of this type are connected to a common transmission line, several transmission lines connected in parallel then constituting a data transfer bus. To avoid the problems of reflections at the end of the line when a signal is transmitted by one of the couplers, a matching resistance is installed at each end of the line. However, another interference phenomenon occurs which hinders the speed of transfer. The connections of the transmitting circuit to the bus lines present an intrinsic inductance which causes transient oscillatory conditions each time the signal to be transmitted is switched. In order to avoid faulty interpretation of the signal, it is then necessary to wait for sufficient damping of the oscillations to occur before generating the next switching.
FIG. 1 shows a state of the art system in which several integrated circuits 1, 2, 3, 4 communicate with each other through the intermediary of a data bus. Such a system is not limited just to the four integrated circuits shown, and is worthwhile as soon as communication is needed between two integrated circuits. The data bus comprises at least one line 51 connecting the different integrated circuits which are to communicate with each other. To exchange n signals simultaneously between the different integrated circuits, the data bus therefore comprises n identical lines 51, each of which is assigned to a signal. For each signal exchanged with the set of other integrated circuits, an integrated circuit has an input-output coupler connected to the line 51, assigned to this signal, through a conductor 5. The coupler comprises a transmitter 6 whose output is connected to the conductor 5, and a receiver 7 whose input is connected to the same conductor 5. The signal transmitted is a binary signal having two states E1 and E0, to each of which corresponds a value of the potential of the line 51. The variations of the potential between these two values are made at high frequency. At these frequencies, the intrinsic inductance and capacitance of the line mean that the variations in potential are propagated in the form of a wave. The propagation velocity of this wave along each section of the line is a function of the inductance and the capacitance of this section, which therefore define a characteristic impedance for the section. Any variation in impedance causes a variation in the propagation velocity of the wave, which then decomposes into a transmitted wave and a reflected wave. To avoid this phenomenon, the line has a structure, and therefore a characteristic impedance Z, which are as far as possible constant over the whole of its length. The connection of a coupler to the line introduces a discontinuity. However, it is usually considered that the structure of the line remains constant over the whole of its length once the couplers are spaced uniformly apart at fixed intervals. It is then sufficient to consider as elementary section a section of the line whose length is equal to one interval. The introduction of a coupler is comparable in essence to the introduction of an additional capacity in the line, resulting in a reduction of the characteristic impedance Z and a slight increase in the propagation time. At the end of the line, one end of an impedance 56 is connected, the other end of the impedance 56 being connected to a fixed potential source Vtt provided by a node 50. The value of the impedance 56 being equal to the characteristic impedance of the line, this enables the echo phenomena to be avoided which are caused by reflection at the end of the line. It follows that, in the absence of transmitted signals, the line 51 is maintained uniformly at the reference potential of the node 50 through the impedance 56.
The transmitter 6 comprises a switch 63 connected between the conductor 5 and a potential source Vn provided by a node 51. This switch is controlled by a circuit 62. Essentially, this circuit receives two signals e and i from a logic unit 8 situated in the same integrated circuit as the transmitter 6. For example, the logic level 1 of the signal e represents an authorisation to transmit for the transmitter 6 and the logic level 1 of the signal i represents the state E1 of the signal to be transmitted. The conjunction of the signals e and i switches the switch 63 on. At any given moment, the authorisation to transmit is limited to a single integrated circuit and no other integrated circuit can transmit on the line 51.
In the integrated circuit which receives the authorisation to transmit, when the signal i is at zero representing the state E0 of the signal to be transmitted, the switch 63 is off. The value of the potential of the conductor 5 is equal to the potential Vtt of the node 50. The signal i switching to 1, representing the state E1 of the signal to be transmitted, switches the switch 63 on. When the switch 63 is on, it presents its own characteristic impedance R3. Thus the on state of the switch 63 causes a current I to appear in the conductor 5 which places the conductor at a potential value Vb whose difference from the potential Vn is equal to the product of the impedance R3 multiplied by the current I. The characteristic impedance of the switch 63 is usually small compared to the characteristic impedance Z of the line, so that the potential Vb is close to the potential Vn of the node 61. The potential Vb of the conductor 5 then propagates along the line 51, towards the couplers of the other integrated circuits connected to the same line 51. Then, the signal i switching to 0, representing the state E0 of the signal to be transmitted, switches the switch 63 off. The known properties of a switch are that when it is off, its characteristic impedance has an infinite resistive component and a slightly capacitive reactive component. Accordingly, the off state of the switch 63 prevents the current I flowing in the conductor 5 and takes the conductor to a value of potential higher than Vb to cancel the current I. The increase of potential of the conductor 5 is then propagated along the line 51 towards the couplers of the other integrated circuits connected to the same line 51.
Each coupler comprises a receiver 7 consisting essentially of a differential amplifier 72. The negative input of the differential amplifier 72 is connected to the conductor 5 and the positive input is connected to a potential source Vref provided by a node 71. The output of the differential amplifier 72 sends a signal s to the logic unit 8 of the integrated circuit to which the coupler belongs. The value of the potential Vref is included between the values Vb and Vtt. For example, if Vtt=1.2 V, Vb=0.4 V, it is possible to have Vref=0.8 V. Accordingly, if the potential of the conductor 5 is close to Vtt, its value greater than the potential Vref switches the differential amplifier 72 to negative saturation, generating a signal s at the logic level 0. If the potential of the conductor 5 is close to Vb, its value lower than the potential Vref takes the amplifier 72 to positive saturation, generating a signal s at the logic level 1.
Thus, each receiver 7 transmits a signal s at level 1 to the logic unit 8 of the integrated circuit to which it belongs when the potential of the conductor 5 connecting to the line 51 is at Vb, that is to say when the state E1 of a signal is transmitted on this line. For example, the receiver 7 transmits a signal s at 0 to the logic unit 8 of the integrated circuit to which it belongs when the potential of the conductor 5 connecting it to the line 51 is at Vtt, that is to say when the state E0 of a signal is transmitted on this line or in the absence of any transmitted signal, which is identical to the state E0.
As we have seen above, the impedance 56 installed at the end of the line with a value equal to the characteristic impedance of the line enables echo phenomena to be avoided. It is therefore not necessary to await the damping of the echo on the line following a change of state of the transmitted signal in order to evaluate the new state. Once the evaluation of the new state is made, a new change of state can then be transmitted if necessary. The absence of any reflected wave from the end of the line is promising regarding the speed of exchange of signals between the different couplers. For example, it might be thought possible not to wait for the voltage wave, related to the change of state, to have reached the end of the line in order to transmit after it a voltage wave related to a fresh change of state of the signal. If sufficiently fast switches are available, it might even be possible to contemplate frequencies of variation of the signals so high that the line 51 would not necessarily be equipotential, but would merely have a defined potential at the point where the signal is evaluated.
However, it has been noted that the voltage wave which propagates along the line 51 from the coupler which transmits the signal does not consist of a perfect step which ideally would be produced by opening and closing of the switch 63. Oscillatory waves are observed which are superimposed upon the perfect step wave, representative of the signal to be transmitted. These oscillatory waves, of transient nature, are damped by the matching impedance at the end of the line. However, it often happens that the amplitude of these waves exceeds the high and low detection thresholds of the receiver 7 connected to the line. It then becomes necessary to await sufficient damping of the oscillations to make a correct evaluation of the signal. It sometimes happens that this waiting time is of a comparable order of magnitude to, or perhaps even greater than, the time required for the voltage waves to reach the end of the line. This observation dashes the hopes of rapidity which cancelling the echo from the end of the line promised.
The problem which is posed for increasing the speed of transfer of signals between the different integrated circuits is to reduce interfering oscillations to the extent that they are practically eliminated. Now, the disadvantage mentioned above is attributed to limits which are inherent in the switching of the switch 63 of the transmitter. In fact, the conductor 5 of each coupler cannot be physically perfect. The physical connection of the coupler, integrated within the circuit whose signal it transmits, to the line 51 which is external to the circuit, gives the conductor 5 an inductance which however low it may be, nonetheless makes its presence felt on the variations of current flowing. In addition, the amplifier 72 of the receiver 7 has an input capacity connected to the conductor 5 which then constitutes an oscillatory circuit.
One solution would be to eliminate the interferences caused by the oscillatory circuit mentioned above at their source by making the conductor 5 or the switch 63 of sufficiently high resistance to create a damping coefficient. This solution is unsatisfactory since it would reduce the pass band of the coupler and induce a Joule effect producing heat in the circuit. In addition, the oscillatory nature of the conductor 5 is not limited only to the coupler which is transmitting but also makes its presence felt at each coupler which receives the signal. Now, if the conductor 5 were resistive, its connection to the line 51 would cause a discontinuity in the impedance of the line induced by the current-voltage wave splitting between the conductor 5 and the line down stream. Also, the conductor 5 of a coupler which is receiving behaves like a capacity in series with an inductance and therefore forms an oscillatory circuit. A modification of the voltage wave in the conductor 5 of the receiver would be likely to be sent back onto the line, thus producing noise.
Another solution would be to provide an impedance for matching to the line 51 at the conductor 5 of the receiving coupler to avoid any retransmission of the wave onto the line from the receiver. However, the presence of a matching resistance at a point other than the end of the line presents a disadvantage for the wave representing the signal. In fact the matching resistance reduces the transmission of the wave beyond the point where it is positioned on the line. It then becomes problematical to install a receiver beyond the first receiver without attenuation of the transmitted wave. This is why this solution is adopted, according to the state of the art, in the form of a link called a series link, where only two transmitter-receiver couplers exchange their signals on one line, each coupler being positioned at the end of the line. In order for an integrated circuit to communicate with a plurality of other integrated circuits, it is then necessary to provide a plurality of transmission lines with a coupler dedicated to the line in each case. Among others, this has the disadvantage of multiplying the number of links between integrated circuits and of requiring it to be defined which of the integrated circuits are to communicate with which others at the time of design of the overall electrical circuit into which these circuits will be incorporated. Clever architecture enables the number of links to be optimised, but degrades the system physically produced in its original design. In spite of all the power of the software it is not possible to develop such a system to make two integrated circuits communicate directly by later creating links which were not produced initially.